1. Field of the Invention
The invention relates to electrolytic reactors. More specifically, the invention relates to an electrolytic reactor containing a series of bipolar element cells that optionally contain separation and additional electrode elements.
2. Description of the Related Art
A conventional electrolysis cell C is represented in the cross section sketch of FIG. 1A. The cell is essentially made up of two metal electrodes 101 and 102, of generally extended surface, submerged in a conductive fluid (electrolyte) 103. The fluid is polarized in such a way as to maintain a constant electric field between the electrodes. Due to the electric field, each ion in the electrolyte migrates towards the electrode polarized with an opposite sign, interacting with the surface of said electrode, and thereby giving rise to an electrochemical transformation of matter. The system is contained in a vessel 104 and sometimes includes a separator 105 to collect the gaseous products produced by the chemical process along the electrodic surface. The separator 105 must be permeable to the electric charge carriers within the electrolyte.
The amount of matter converted in the unit time depends upon the D.C. flow between the electrodes and the electrode surface area. When an increase of such amount is desired and a further increase of the electrodes surface is impracticable, it is customary to assemble a certain number of cells in series or in parallel. The series configuration gives rise to the well-known bipolar electrolyzers, corresponding to the sketch shown in FIG. 1B. In these apparatus cells C.sub.0, C.sub.1, . . . C.sub.i, . . . C.sub.n are separated from each other by conductive elements E.sub.1, E.sub.2, . . . E.sub.i, . . . E.sub.n carrying opposite polarity on the two opposite faces, i.e. F.sub.1, F.sub.2, in contact with two consecutive cells. These elements are bipolar plates. FIG. 1B separators S.sub.i for the electrodic compartments. Electrolyzers corresponding to this general type are used in industrial processes, such as caustic-chlorine production and water electrolysis.
The quantity of electric work spent in order to carry out the electrochemical process is higher than the amount thermodynamically necessary. This is due to energy losses in non-reversible phenomena. Some of these are: the electrical current dispersion along the electrolyte feeding and discharge piping which is also a source of localized corrosion of the electrolysis equipment; the voltage drop across the electrolyte and the separator; the presence of gas bubbles between the electrodes; and, the ohmic drops due to the current flow throughout the structure of the cells. Other losses are independent of the cell geometry. These depend on the nature of the electrodic surfaces in relation to the specific electrochemical process. Their amount is measured by the so-called electrode overvoltage. An important factor in their reduction is the opportunity the electrolysis cells have to operate at relatively high temperature. Presently, commercial cells are limited in their operating temperature by the presence of parts made of materials which are not able to resist high temperatures. Operation at high temperature is often conducted at superatmospheric pressure to prevent the liquid electrolysis from boiling, or at least restrict its evaporation. Commercial cells offer only limited possibilities to operate under pressure.
The current losses throughout the liquid connections with the external circuits are reduced by minimizing the cross section of the liquids' distribution ducts. This requires an increase of pressure drop not applicable to all the electrolysis equipment where the electrolyte circulation is effected by thermosiphon procedures. The reduction of the gas ducts' cross section requires in turn the reduction of the specific volume of the gaseous phase. This means operating under pressure, with the above-mentioned limitations.
The energy losses due to the D.C. field crossing the electrolyte flow can be lowered by shortening, as far as possible, the mutual distance between the electrodes. Almost all commercial electrolyzers have electrodes at a distance from a few millimeters up to some centimeters from each other. The reciprocal location of the electrodes gives problems for their parallelism which are difficult to solve, particularly if the electrodic surface is large. Lack of parallelism causes higher current flows where the distances are short, with local increases in current density and consequent extra losses of energy due to the excessive overvoltage and ohmic drop. Moving the electrodes closer to the separator tends to trap the gases evolved by the electrochemical reaction between the electrodes and the separator, thereby interrupting the electrical current flow.
The general lack of design simplicity must be added to all the above-mentioned limitations of commercial electrolyzers. This means higher costs, difficult access to the various parts, assembling and disassembling, replacement of worn out elements such as separators or the electrodes to be catalytically regenerated, and peripheral gas- and liquid-tight cells for atmospheric pressure operation.
Finally, limited usable current density necessitates large electrodic surfaces, with resulting high costs and large horizontal and vertical size of the electrolysis equipment.